Examples of prior art voltage regulators are described for example in U.S. Pat. Nos. 6,229,292 B1 or 6,069,471.
Such a voltage regulator is described for example in U.S. Pat. Nos. 6,229,292 B1 or 6,069,471.
In the case of such voltage regulators, the converter unit provides an output current which brings about a voltage drop across the parallel circuit formed by the output capacitor and the load, said voltage drop corresponding to the output voltage. Via the feedback path, load-dictated fluctuations in the output voltage are readjusted in a known manner in that, in the event of a decrease in the output voltage, the power consumption of the regulator is increased and the average output current is increased and in that, in the event of a rise in the output voltage, the power consumption of the regulator is reduced and the average output current is reduced. In the voltage regulator of the generic type, the output capacitor serves as a buffer for reducing fluctuations in the output voltage in the case of load changes of the load connected to the output terminal and in particular in the case of converter units with a switching converter for smoothing the output current.
Since the converter unit can react to load changes only with a time delay, abrupt load changes lead to momentary fluctuations in the output voltage, as is illustrated with reference to FIGS. 1 and 2. In FIG. 1, AK designates the output terminal of a converter unit of arbitrary configuration, which provides an output current Iout for a parallel circuit formed by a buffer capacitor 20 and a load, said output current Iout bringing about a voltage drop Vout across the parallel circuit. FIG. 1 shows the electrical equivalent circuit diagram of the output capacitor 20, which comprises a capacitor component, represented by the capacitor C, and a resistive component ESR connected in series with the capacitive component C. In this case, the resistive component ESR takes account of unavoidable conduction losses of a real capacitor.
Consideration will now be given to the case illustrated in the figure, in which, in the open-circuit case, an output voltage Vout having a nominal value Vout_nom is set, no current or only a very small current being required to maintain said output voltage. If, at an instant t0, the current consumption I of the load rises rapidly owing to a load change, then this current requirement can initially be covered only by the output capacitor 20, the current which is drawn from the capacitor 20 and initially corresponds to the load current now taken up bringing about, across the equivalent resistance of said capacitor, a voltage drop ΔU resulting from the product of the resistance ESR and the current change ΔI (in the present case ΔI=Imax). The output voltage Vout thereby decreases by the value ΔU. The power consumption of the converter unit is thereupon readjusted until the output current Iout is adapted to the changed load conditions and the nominal voltage Vout_nom is again present at the output. If, at an instant t1, the current consumption of the load falls from the value Imax to zero owing to an open circuit, then only the output capacitor 20 is momentarily able to take up the output current adapted to the load conditions prevailing until then, which leads to an output voltage increased by the voltage drop ΔU=ΔI·ESR=Imax·ESR. The output current is thereupon readjusted until the output current Iout is zero.
To summarize, in the case of a maximum current consumption of the load of Imax, fluctuations in the output voltage Vout of Δout=2·Imax·ESR can thus occur.
In order to reduce the fluctuations in the output voltage, Ron Lenk: “Understanding Droop and Programmable Active Droop”, Application Bulletin AB-24, Fairchild Semiconductor, FIG. 3, discloses connecting a resistor downstream of the output of the converter, across which resistor the output current likewise brings about a voltage drop and which resistor, in the event of a change in the current consumption of the load, accepts a part of the resultant voltage fluctuations, so that the actual output voltage corresponds to the nominal value of the output voltage minus the voltage drop across the resistor.
In the abovementioned publications U.S. Pat. Nos. 6,229,292 B1 and 6,069,471, in order to reduce such voltage fluctuations in the output voltage, provision is made for reducing a reference value Vref1, depending on which the output voltage or the output current is set, in accordance withVref1=Vref−I·ESR  (1),where Vref is a constant reference value, I is the load current and ESR is the equivalent resistance. In the case of a large current requirement of the load, which, in the case of a load change, would bring about a correspondingly large change in voltage across the equivalent resistance, the reference value Vref1 is in this case reduced in order to correspondingly reduce the output voltage and the output current and thereby to reduce the fluctuation range of the output voltage in the case of a load change.
It is an aim of the present invention to provide a voltage regulator having an output capacitor connected to an output terminal and having an equivalent resistance in which a fluctuation range of the output voltage in the case of load changes of a load connected to the voltage regulator is reduced.